In tiltable windows to be mounted in a sloping roof, the joints between the lateral members of the sash and frame are usually covered with cover rails, each comprising an upper section secured to the frame and a lower section secured to the movable sash, and in order to achieve a sufficient tightness in the particularly critical area of the hinge mountings, the two sections generally overlap each other over a suitable distance in the closed position of the window. To make this possible, the movement of the window sash, at any rate in the vicinity of its closed position, takes place as a swinging about an axis spaced outwardly from the window and at a distance therefrom depending on the desired degree of overlapping and on the distance between the cover rail sections in the overlapping area. The axis can be established by means of pivot pins interconnecting protruding parts of the frame and the sash, respectively, but this is generally best avoided, inter alia because the parts and pins in question would be exposed to climatic influences; it has been found preferable instead to establish the axis by means of mountings built into the frame and sash and having associated arc-shaped sliding rails and guides.
A known hinge device of this type, see e.g. British Patent Specification No. 1,028,251, comprises a frame part having an arc-shaped guide firmly secured to a base plate, and an associated sash part having a base plate which through a pivot pin is swingably connected with a bowl-shaped sector, the edge portion of which constitutes the sliding rail cooperating with the guide of the frame part. At the beginning of the opening movement of the window, the sliding rail is displaced in the guide until the pivot pin is positioned clearly outside the plane formed by the outer side of the frame, whereafter the further opening movement takes place by swinging of the sash about this pivot pin.
Regardless of the manner in which the externally positioned pivotal axis for the initial opening movement of the sash is established, it must in practice necessarily be placed at some distance, in horizontal direction, from the horizontal axis of gravity of the sash, for which reason the sash when released from its closed position will tend to move towards the position in which its axis of gravity is positioned vertically under the pivotal axis. This tendency can be counteracted by providing a suitable friction between the guiding surfaces sliding on each other, but such friction will, for one thing, obstruct the opening and the closing of the window in other situations and, for another thing, it will generally now and then require some adjustment on account of variations due to wear, dirt and the like.